1. Field of the Invention
This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hot gas flow path through a gas turbine engine. More particularly, this invention is directed to a coating composition that exhibits improved high temperature stability when used to protect a silicon-containing substrate.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys. While superalloys have found wide use for components throughout gas turbine engines, alternative materials have been proposed. For example, composite materials, particularly silicon-based composites with silicon carbide (SiC) as a matrix and/or as a reinforcing material, are currently being considered for high temperature applications, such as combustor liners, vanes, shrouds, airfoils, and other hot section components of gas turbine engines.
In many high temperature applications, a protective coating is beneficial or required for a Si-containing material. For example, protection with a suitable thermal-insulating layer reduces the operating temperature and thermal gradient through the material. Additionally, such coatings should provide environmental protection by inhibiting the major mechanism for degradation of Si-containing materials in a corrosive water-containing environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH)4) products. Consequently, besides low thermal conductivity, stability is a critical requirement of a thermal barrier coating system for a Si-containing material in high temperature environments containing water vapors. Other important properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the SiC-containing material, low permeability for oxidants, and chemical compatibility with the Si-containing material and silica scale formed from oxidation. As a result, suitable protective coatings for gas turbine engine components formed of Si-containing materials essentially have a dual function, serving as a thermal barrier and simultaneously providing environmental protection. A coating system having this dual function may be termed a thermal/environmental barrier coating (T/EBC) system.
Various single-layer and multilayer T/EBC systems have been investigated for use on Si-containing substrates. Coatings of zirconia partially or filly stabilized with yltria (YSZ) as a thermal barrier layer exhibit excellent environmental resistance. However, YSZ does not adhere well to Si-containing materials (SiC or silicon) because of a CTE mismatch (about 10 ppm/xc2x0 C. for YSZ as compared to about 4.9 ppm/xc2x0 C. for SiC/SiC composites). Mullite (3Al2O3.2SiO2) has been proposed as a bond coat for YSZ on Si-containing substrate materials to compensate for this difference in CTE (mullite having a CTE of about 5.5 ppm/xc2x0 C.). Barium strontium aluminosilicate (BSAS; (Ba1-xSrx)Oxe2x80x94Al2O3xe2x80x94SiO2) has also been proposed as a bond coat for YSZ in U.S. Pat. No. 5,985,470 to Spitsberg et al., U.S. Pat. No. 6,299,988 to Wang et al., and U.S. Pat. No. 6,444,335 to Wang et al., which are assigned to the assignee of the present invention. BSAS and other alkaline earth aluminosilicates have also been proposed as protective coatings for Si-containing materials in view of their excellent environmental protection properties and low thermal conductivity.
For example, U.S. Pat. Nos. 6,254,935, 6,365,288, 6,387,456, and 6,410,148 to Eaton et al. disclose the use of BSAS as outer protective barrier coatings for Si-containing substrates. U.S. Pat. No. 6,410,148 also discloses barium oxide (BaO) and barium aluminosilicates (BAS) as suitable barrier coatings for Si-containing substrates.
Stoichiometric BSAS (0.75BaO.0.25SrO.Al2O3.2SiO2, or an alkaline-earth:aluminosilicate molar percentage ratio of about 25:75) has been reported as the preferred alkaline earth aluminosilicate composition, examples of which include U.S. Pat. Nos. 6,254,935, 6,365,288, 6,387,456, and 6,410,148. Non-stoichiometric BSAS compositions have also been proposed. For example, U.S. Pat. No. 6,352,790 to Eaton et al. discloses a barrier layer formed of a mixture of an alkaline earth aluminosilicate (e.g., stoichiometric BSAS) and an additive, disclosed as being alumina (Al2O3) or a phase formed of alumina and either barium oxide or strontia (SrO). The resulting barrier layer is referred to as a non-stoichiometric BSAS, BAS or strontium aluminosilicate (SAS) as a result of an increased alumina content and a sub-stoichiometric silica (SiO2) content. However, the barrier layer is not disclosed as being a homogeneous, single-phase composition, but is instead an aluminosilicate in which are dispersed phases formed by reaction of the additive with free silica that deposits or forms in the barrier coating.
Notwithstanding the above-noted advances in T/EBC materials for Si-containing substrates, further improvements in coating life would be desirable. In particular, longer exposures at temperatures sustained in the combustion environment of a gas turbine engine (e.g., high pressure steam and high gas velocities) have resulted in the volatilization of existing BSAS materials, causing coating recession that ultimately leads to degradation of the environmental protective properties of the coating. Therefore, it would be desirable if coatings for silicon-containing substrates were available that exhibit still lower recession rates.
The present invention provides a coating composition for use in a thermal/environmental barrier coating (T/EBC) system particularly suited for protecting silicon-containing substrates, such as articles exposed to high temperatures including the hostile thermal environment of a gas turbine engine.
The coating composition of this invention contains barium oxide, strontia, alumina and optionally silica in molar percentage ratios different than that of stoichiometric BSAS (18.75:6.25:25:50). More particularly, if silica is omitted, the coating composition is not an alkaline earth aluminosilicate, but an alkaline earth aluminate. If silica is present, the Al2O3+SiO2 content in the coating composition is about 50 to 63 molar percent (compared to about 75 molar percent for stoichiometric BSAS), resulting in a BaO+SrO content of about 37 to 50 molar percent (compared to about 25 molar percent for stoichiometric BSAS). While not wishing to be held to any particular theory, it is believed that the relatively higher BaO+SrO content and relatively lower Al2O+3SiO2 content in the coating composition renders the composition more stable over long exposures at high temperatures than stoichiometric BSAS.
Five particular phases have been identified whose high temperature stability has been concluded to be better than that of stoichiometric BSAS. One such phase contains, by molar percent, about 37% to about 39% barium oxide, about 9% to about 11% strontia, about 47% to about 49% alumina, and up to about 5% silica, yielding a molar ratio of barium oxide, strontia, alumina and silica of about 0.8:0.2:1:0.1, respectively. Therefore, both the BaO+SrO content and the Al2O3+SiO2 content in the composition is about 50 molar percent. Another phase identified with this invention contains, by molar percent, about 27% to about 28% barium oxide, about 9% to about 10% strontia, about 31% to about 32% alumina, and about 31% to about 32% silica, yielding a molar ratio of barium oxide, strontia, alumina and silica of about 1.3:0.4:1.5:1.5, respectively. Therefore, the BaO+SrO and Al2O3+SiO2 contents in the composition are about 37 and 63 molar percent, respectively. Broad compositional ranges for the coating composition of this invention are, by molar percent, about 20% to about 40% barium oxide, about 9% to about 20% strontia, about 19% to about 50% alumina, and up to about 40% silica.
A coating system incorporating the coating composition of this invention preferably includes a bond coat between the coating composition and the silicon-containing surface, by which the coating composition is adhered to the surface. Suitable bond coats may comprise one or more layers of silicon, mullite, stoichiometric BSAS, and mixtures of mullite and stoichiometric BSAS. To increase its allowable outer surface temperature, the coating system may further include an outermost coating on and contacting the coating composition. Suitable materials for the outermost coating include stabilized zirconia or another high-temperature ceramic material.
Compositions for the coating composition of this invention are generally characterized by aluminum-rich and strontium-rich ceramic phases. These phases are believed to contribute to the ability of the coating composition to exhibit a lower recession rate in the combustion environment of a gas turbine engine. While non-stoichiometric in terms of alkaline earth aluminosilicate compositions used in the past, the coating composition of this invention is chemically similar to stoichiometric BSAS. As a result, the coating composition can be deposited on stoichiometric BSAS, with the expectation that a strong chemical bond will exist therebetween that promotes the mechanical integrity of the coating system.
Other objects and advantages of this invention will be better appreciated from the following detailed description.